1. Field of the Invention
This invention relates generally to methods and apparatus for conditioning an analog input signal, and specifically to limiting or xe2x80x9cclippingxe2x80x9d an analog signal that is input to an analog-to-digital converter, the input signal being variant in voltage with time.
2. Description of Related Art
Analog circuits are commonly used to condition signals supplied to or received from other devices or functional units within an integrated device. For example, an analog circuit is often used as a xe2x80x9cfront endxe2x80x9d signal conditioner for an analog-to-digital converter (ADC) implemented in the form of an application-specific integrated circuit (ASIC), the ADC converting the conditioned analog signal to a binary digital format for use by digital circuitry either within or external to the ASIC. In the case of an ADC, it is often necessary to condition the analog input signal in order to obtain improved or even acceptable performance from the ADC, as described in greater detail below. While the following exemplary discussion is cast in terms of an ADC/ASIC, those skilled in electronic art shall appreciate that analog signal conditioning may be utilized in a broad range of electronic applications.
Analog-to-digital converters are well known in the electronic arts. ADC devices take an analog input signal of varying voltage and convert it to a binary digital representation of the input signal for subsequent processing by digital circuitry such as a digital signal processor (DSP). ADC devices can generally be divided into different functional categories, including xe2x80x9cover-samplingxe2x80x9d ADC devices. Over-sampling converters, as the name implies, sample the analog signal at a frequency that is typically much higher than the Nyquist frequency. The delta-sigma converter (also referred to as a xe2x80x9csigma-deltaxe2x80x9d converter) is one type of over-sampling converter that is commonly used in applications where the high sampling rate provides intrinsic benefits. Such applications include digital audio and video decoding. As is well known, Delta modulation refers generally to the process whereby the digital output signal represents the change, or xe2x80x9cdeltaxe2x80x9d, of the analog input signal. Delta-sigma converters integrate the analog input signal before performing delta modulation. Hence, the integral of the analog input signal is encoded in delta sigma converters. In contrast, only the delta or change in the input signal is encoded in the simple delta modulator. A digital sample rate reduction filter (commonly known as a decimation filter) is also commonly used to provide an output sampling rate that differs from the Nyquist frequency of the signal. The combination of the over-sampling process and the decimation process produces greater resolution than a typical Nyquist converter.
Third order and higher order delta-sigma converters, in contrast to their lower-order counterparts, provide enhanced performance due to their ability to more effectively remove in-band noise from the signal. Hence, a third-order or higher order delta-sigma converter will provide a higher quality digital audio or video signal (i.e., higher Signal-to-Noise Ratios (SNR)) than second-order or lower order counterparts.
Despite their enhanced performance and utility in certain applications, all third order (and higher order) delta sigma converters are inherently unstable. This instability arises from, inter alia, the noise transfer function (NTF) associated with the converter. Typically, this instability is manifested in very harsh and largely unpredictable signal degradation when the relevant threshold condition (ie., input signal voltage) is exceeded. Throughout the remainder of this specification the term xe2x80x9cexceedxe2x80x9d or xe2x80x9cexceedsxe2x80x9d is used to describe the condition when the input signal voltage level is either greater than a high threshold voltage or less than a low threshold voltage.
Unlike other types of circuits that may exhibit more xe2x80x9cgracefulxe2x80x9d degradation (e.g., a progressively increasing noise component or distortion present in the output signal) as the threshold voltage is exceeded, third and higher order delta-sigma converters tend to degrade catastrophically. Even small increases in the input voltage above a threshold induce large oscillations within the circuit. This results in an output signal that is almost entirely dominated by noise, and that bears little or no resemblance to the input signal. This type of behavior is especially troubling in applications in which it is desirable to have improved control over the degradation of the output signal, such as in digital audio applications.
Consider, for example, the use of a third-order or higher order delta-sigma ADC in a digital wireless telephone wherein there are no limitations placed on the input signal that is applied to the ADC. When a caller""s audio input produces input voltages that are less than the specified threshold value, the noise component within the output signal of the ADC is minimized, and the useful signal is maximized. However, when the input signal exceeds a level that induces oscillation of the converter, there is a rapid and often complete degradation of the signal. In such cases, a very abrupt cessation of voice may become manifest and perceived by the listener. This cessation may be followed by an unintelligible string of voice information until the signal level falls very near or below the threshold value of the ADC. Clearly such circuit behavior is unacceptable and must be avoided.
While third-order and higher-order converters can be made conditionally stable by appropriately restricting the input signal voltage or via system level design, such design and operational restrictions place a significant burden on the system designer. This is highly undesirable from the perspective of labor and man-hours required to implement the restrictions, thereby potentially increasing required die area, external component costs and time-to-market of devices using delta-sigma converters. In many applications, such design restrictions are exceedingly difficult to implement, such as in the case of a tuner circuit whose output (ie., the input to the ADC) may vary hundreds of millivolts. Furthermore, prior art approaches for restricting voltages that are provided as input to the ADC can have significant deleterious effects on the quality and useful range of the input signal.
Some techniques for restricting or conditioning voltages of an analog signal that is input to another device, such as a higher-order ADC, have been proposed in the prior art. These techniques typically require that the input signal voltage be progressively restricted as it approaches a threshold value of interest. For example, one approach utilizes discrete components, such as diodes, to xe2x80x9cclipxe2x80x9d an input voltage as it approaches a pre-determined threshold voltage. The degree of signal clipping is substantially dependent upon the proximity of the input signal voltage to the pre-determined threshold. At a voltage that is substantially distant from the threshold voltage, there will be very little if any clipping of the input signal. However, as the input signal voltage approaches the threshold voltage, more clipping is applied until the input signal is completely clipped so as to maintain its voltage at or below the threshold level. When completely clipped in this fashion, no amount of increase in the input signal voltage will drive the output voltage to a level that is higher than the threshold voltage.
While effective at clipping the signal so as to avoid exceeding the threshold, the foregoing technique suffers from the significant disadvantage of distorting the input signal when it operates within the voltage range of interest. The degree of signal distortion varies depending on the proximity of the voltage to the threshold. The diodes used by the previous clipping techniques create increased signal distortion as the voltage thresholds are approached. At some point the distortion becomes sufficiently significant such that the resultant signal is no longer useful. At this point, the ADC device range that causes oscillation may not be reached. Hence, if the useful range of voltages for the unclipped signal is limited (as limited by the design goal of avoiding oscillations within the higher-order delta sigma ADC), the useful range of the progressively xe2x80x9cclippedxe2x80x9d signal is also limited. The useful range of the signal is therefore disadvantageously unnecessarily restricted by the prior art progressive clipping approach.
In addition to distorting the input signal in the regions adjacent to the voltage thresholds, the prior art techniques also distort the input signal throughout its entire useful range. This latter distortion stems from the fact that the diodes (or other components that are used to clip or condition the signal) are always maintained directly in the signal path between the input and the receiving device (e.g., ADC). Even when no clipping occurs, the analog input signal is somewhat distorted because the signal is always passed through the diodes.
In addition to efficiently and noiselessly clipping or conditioning an input signal, it is also desirable to minimize power that is consumed by the signal conditioning circuitry. Benefits of reduced power consumption include, inter alia, increased power source longevity (such as batteries used in a portable device), and reduced heat generation. Power is consumed within integrated circuits by a variety of different components and the operations performed by those components. In many analog circuit designs, some minimal current flow must always be provided to the components of the circuit (such as operational amplifiers) in order to maintain the components and circuit in a desired operational state. This current flow is required even when the circuit or portions thereof are not in use. Such indiscriminant current flow increases the power consumption of the circuit and its integrated circuit, which is highly undesirable. Therefore, it is desirable to provide a signal conditioning apparatus that not only reduces signal distortion, but also reduces the power consumed by circuit components, especially when the signal conditioning apparatus is not actively clipping the input signal.
Based on the foregoing, an improved method and apparatus for conditioning an analog signal that is subsequently input to downstream devices, such as higher order delta-sigma ADCs, is desired. Such an improved method and apparatus should maximize the useful range of the input signal consistent with the limitations of the downstream device, while also mitigating or eliminating distortion of the signal within the useful range. Such an improved method and apparatus should be implemented so as to consume a minimum amount of power necessary for operation, thereby increasing power efficiencies and reducing heat generation associated thereto. Lastly, the improved method and apparatus should also ideally be capable of being implemented in silicon, so as to facilitate use within an integrated circuit (IC) such as an ASIC.
The present invention satisfies the aforementioned needs by providing an improved method and apparatus for conditioning an analog input signal for use by downstream devices.
In one exemplary embodiment, the apparatus comprises an analog conditioning circuit having (a) a reference voltage generator circuit; (b) voltage comparators; (c) threshold voltage generator circuits; and (d) a plurality of transistor gate switch limiters. The reference voltage generator circuit generates both low and high threshold voltages using a voltage divider network. The voltage of the input signal is compared against the threshold voltages. Comparison of the input signal to the threshold voltages is performed by the voltage comparators. When the input signal voltage is either greater than the low threshold voltage, or less than high threshold voltage (i.e., when the input operates within the desired dynamic operating range and therefore does not xe2x80x9cexceedxe2x80x9d either of the threshold voltages), all conditioning circuit components are effectively removed from the circuit by the switch limiters. In this mode of operation, the conditioning circuit is transparent to the input signal, and advantageously no distortion of the input signal occurs within the dynamic range of operation. However, when the input signal meets or exceeds one of the threshold voltages (i.e., when the input signal is either equal to or less then the low threshold voltage, or equal to or greater than the high threshold), the conditioning circuit components are placed in the signal pathway by the switch limiters and are thereby used to xe2x80x9cclipxe2x80x9d the input signal.
Specifically, when the present inventive signal conditioning apparatus is in a xe2x80x9cclippingxe2x80x9d mode of operation, the output of the threshold voltage generator exceeded by the input signal is applied to the output of the conditioning circuit. The resulting output signal therefore never exceeds the relevant threshold voltage, and the input signal is thereby appropriately xe2x80x9cclippedxe2x80x9d. The conditioning circuit is designed such that a very rapid transition between the unclipped and clipped states is provided, thereby effectively eliminating distortion of the input signal within the dynamic range of operation. This transition is effected in part by using MOSFET-based transistor gates within the switch limiters.
An improved method of conditioning an analog signal is also disclosed. The method generally comprises the steps of providing an analog signal whose voltage varies as a function of time; defining at least one voltage threshold for the analog signal; monitoring the relationship of the analog signal with respect to the voltage threshold; and selectively inserting at least one voltage conditioning component in the signal path of the analog signal to effect the signal conditioning desired. In one exemplary embodiment, the method is applied using the inventive analog conditioning circuit to clip the analog signal before it is input to a high-order delta-sigma ADC. Two thresholds are defined for the signal (i.e., upper and lower voltage thresholds), corresponding generally to the upper and lower dynamic range limits of the ADC. As the input signal voltage approaches either one of the thresholds, the signal is clipped sharply by changing the state of the switch limiters within the conditioning circuit. The change in state of the switch limiters applies the output of the threshold voltage generators (specifically, the output of the threshold generator associated with the threshold that was exceeded by the input signal) to the output of the conditioning circuit (and therefore the input of the ADC). The ADC is thereby prevented from oscillating yet the quality of the input signal is preserved within the dynamic range.
The improved method and apparatus for conditioning an analog signal also reduces power consumption of an analog signal conditioning circuit when signal conditioning is not required. In one exemplary embodiment, the relationship between the analog input signal and the threshold values is determined using voltage comparators that control the operation of the conditioning circuit. When a threshold voltage is exceeded, the comparators generate a signal that results in the introduction of clipped signals into the signal path and ultimately to the output of the conditioning circuit. During periods when the input signal operates below the predetermined thresholds, certain components within the conditioning circuit are rendered effectively inactive using parallel current sources that reduce the current draw (and power consumption) of these components. During these xe2x80x9cinactivexe2x80x9d periods, only a minimal amount of current is provided to reference voltage generators of the conditioning circuit. When the input signal exceeds one of the references thresholds voltages, the current sources provide full power to the reference voltage generators. Power is thereby conserved in the present inventive conditioning circuit when clipping is not required.